Infinite Falling Images
What you want are particles. This will allow you to have may flowers falling, with minimal simulation overhead. The idea is you have the flowers on "cards" The cards are attached to the particles, and the particles physics replicate the falling.

Steps: 


Position you camea
add a plane as wide as the camera in front of it.
Move your flower object to layer 2 M
Add a particle system to the plane, using the particle tab
Particle setup:




Make sure all the circled settings match:

Set the Emit From to Faces, so it comes out of the face of the plane. Verts would have it come out of the corners, where the vertices are and volume would work very well since our plane has no volume.

I set the normal a bit higher to make the flowers a bit faster.

Rotation needs to be set to Velocity hair, so that the planes face the camera, you may have to edit the phase option to the left.

Set the render to Object this will make your particles appear as planes, like you desired. Set the dupli object to match your image of the flower. You can adjust the size to scale the flowers.


Apply these materials to the picture of the flower:




In order to have the transparent background, we need to make a shadeless transparent material. The image will then add alpha back in later.


For the texture use these settings:




Select alpha under influence, then set it to one. The will override the materials alpha.


Render animation.



Note quality has been reduced by high gif compression, it will look fine when rendered

Demonstration: